Chinese authorities have blocked most domestic users from the main Google.com search engine, a media watchdog said.

Internet users in major Chinese cities faced difficulties accessing Google's international site in the past week, Reporters Without Borders said.

But Google.cn, the controversial Chinese language version launched in January, has not been affected.

The site blocks politically sensitive material to comply with government censorship rules.

"It was only to be expected that Google.com would be gradually sidelined after the censored version was launched in January," Reporters Without Borders said in a statement.

"Google has just definitively joined the club of Western companies that comply with online censorship in China," the organisation said.

Google.com, the search engine's uncensored international site, had previously been available to Chinese web users, but problems accessing the site had been reported across the country recently. It was blocked nationwide on 31 May, the statement said.

The blocking was also being extended to Google News and Google Mail, Reporters Without Borders said.

'Principled approach'

A spokeswoman for Google in Beijing said that the problem was under investigation.